238 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



shell, their edges roughened, those of the posterior valve almost 

 crenate. Anterior valve with 11, second valve with 2-1, third 

 to seventh valves with 1-1, posterior valve with 12, slits. 



Girdle wide, regular ; covered dorsally with a closely imbri- 

 cating armature of large, smooth or barely striated, convex 

 scales, each normally bearing a short, striated, nipple-shaped 

 process projecting upward from the dorsal end. 



Color of outer surface of shell a reddish brown of varying 

 intensity in different specimens, sometimes nearly black ; girdle 

 lighter. Interior of shell light salmon. 



Maximum length of the type and largest specimen 29.5 mm. ; 

 width 16 mm. 



Type: A shell preserved dry [S. S. B. 159] as Cat. No. 3700 

 of the author's collection. Paratypes have been deposited in 

 the collections of the California Academy of Sciences, the 

 Academy of Natural Sciences of Philadelphia, the United 

 States National Museum, and the private collection of Mr. 

 George Willett. 



Type Locality: 15-20 fathoms, Forrester Island, Alaska; 

 George Willett, May-July-August, 1914-1916; 36 specimens. 



Remarks: As above noted, this species has a close superficial 

 resemblance to its associate and congener, /. mertcnsii. It is 

 easily recognized, however, by its narrower outline when 

 viewed vertically, the grooves chiseled between the ribs of the 

 terminal and lateral areas, its finer sculpture, and generally 

 characteristic darker red-brown coloration. A nearer relative 

 is possibly a species now being described as /. intcrfossa Berry, 

 from northern Japan, but the more arcuate side slopes, the less 

 pitted sculpturing of the central areas, and again the color, fur- 

 nish easy distinguishing marks. 



13. Ischnochiton (Ischnoradsia) trifidus (Carpenter) 



One specimen of this interesting species was taken at a very 

 low tide near the Waterfall Cannery, Prince of Wales Island. 

 Five others were dredged in 15-20 fathoms at Forrester Island 

 in company with /. mertcnsii. Mr. Willett believes it to be a 

 rather rare species. 



